The genetic and physiological factors governing the expression of virulence factors by Vibrio cholerae and Escherichia coli are under investigation. Comparative studies on the enterotoxins elaborated by these (and other) pathogens are in progress particularly with regard to physical and immunological similarities. The incidence of diarrheal disease due to enterotoxin producing organisms is being determined. One study involves pediatric patients admitted to the hospital and those presenting with diarrhea at the medical center's outpatient clinics. The second study is on the natural history of enterotoxin-induced diarrheal disease in sub-human primates. It is hoped that the latter investigation can be used as a model of the human disease. Laboratory experiments on the organisms issuing from the surveys are aimed at correlating genetic control of virulence factors with their phenotypic expression. The number and size of plasmids in tox plus and tox minus strains are matched with relative toxigenicity, ability to adhere or colonize epithelial surfaces, and a battery of biochemical characteristics. The interaction of bacteriophage specific for tox plus isolates is being studied and it is hoped that eventually a phage-typing system will evolve. The consequence of this research will be easier ways to identify the etiologic agents of diarrheal disease and methods to abrogate their colonization and toxin elaboration in the human host.